1. Field of the Invention
The present invention relates to a scanning exposure apparatus of the so-called slit scan method or step-and-scan method, or the like for continuously exposure-printing patterns formed on a mask onto a photosensitive substrate by illuminating, for example, a rectangular or arcuate illumination area with exposure light and synchronously scanning the mask and photosensitive substrate relative to the illumination area.
2. Related Background Art
Projection exposure apparatus, which are arranged to project (or expose) an image of patterns of a reticle as a mask through a projection optical system onto a wafer (or glass plate, etc.) coated with a photoresist or the like, have been used heretofore in fabricating, e.g. semiconductor devices, liquid crystal displays, image pickup devices (CCDs etc.) and magnetic thin-film heads, by the photolithography techniques. One of basic functions of such projection exposure apparatus is an exposure dose controlling function to maintain an exposure dose (integral exposure energy) at each point in each shot area of wafer within a proper range.
Another demand these days is to further enhance the resolution of patterns printed on the wafer. One of the techniques for enhancing the resolution is to decrease the wavelength of exposure light. Concerning this, light sources that can emit light of short wavelengths, among those presently available as an exposure light source, are laser light sources of the pulse oscillation type (pulsed light sources), for example excimer laser light sources such as a KrF excimer laser or an ArF excimer laser, metal vapor laser light sources, or YAG laser light sources. The pulsed light sources, however, have different characteristics from light sources of a continuous emission type such as a mercury lamp: exposure energy (pulse energy) of pulsed light emitted from the pulsed light source is dispersed within a certain range pulse by pulse. Thus, the dispersion in pulse energy needs to be taken into account for control of exposure dose in the use of the pulsed light source.
When such a pulsed light source is applied to a projection exposure apparatus of a one-shot exposure type such as a stepper, a known control of exposure dose is the so-called cut-off control, in which a so-called integrator sensor is used for continuously monitoring the light amount of exposure light and in which emission from the pulsed light source is repeated until a result of the measurement by the integrator sensor exceeds a specific, critical level determined by taking into account the dispersion in pulse energy with respect to a target exposure dose. Further, another known control is the so-called pulse-by-pulse control, in which the pulse energy is adjusted for every emission of pulse in accordance with the result of measurement by the integrator sensor. This pulse-by-pulse control can decrease a minimum number of exposure pulses per point on the wafer to a number smaller than that in the cut-off control.
Meanwhile, in recent years, the size of one chip of, e.g., a semiconductor device has increased and the projection exposure apparatus has been required to increase the exposure area so as to project patterns with a larger area on the reticle onto the wafer. However, simply increasing an exposure field of projection optical system for that purpose would result in increasing the production cost, because the projection optical system becomes complex in order to keep aberrations within an acceptable range over the entire surface of the wide exposure field, and would result in increasing the scale of the projection optical system, which would in turn increase the size of the entire apparatus too much. Thus, in order to meet the demand to increase the area of transferred patterns while not increasing the size of the exposure field of projection optical system too much, a projection exposure apparatus of the so-called slit scan type, has been developed which is arranged to continuously exposure-print the patterns on the reticle onto the wafer by synchronously scanning the reticle and wafer relative to an illumination area, for example, of a rectangular, arcuate, hexagonal, or another shape (hereinafter referred to as a "slit illumination area"). When exposures are effected on many of shot areas each by the slit scan method, a stepping method is employed for movement between shot areas. The exposure method of this type is thus called a step-and-scan method.
The aforementioned cut-off control cannot be applied to the control of exposure dose in the projection exposure apparatus employing the above scanning exposure method and the pulsed light source, because exposure is continuously performed. The pulse-by-pulse control requires a complex algorithm. Here is explained with an example of a so-called pulse count method for exposing all points on the wafer with a same number of pulses, as the simplest control method of exposure dose in the case of the scanning exposure method. In this pulse count method, an average of present pulse energy is measured before exposure, an energy modulator in an illumination optical system modulates an exposure dose of pulse energy, based on a result of the measurement, and then scanning exposure is effected so as to keep the number of irradiation pulses constant for every point on the wafer.
A precondition for the control of exposure dose in the conventional scanning exposure method as described above is that the average of pulse energy initially measured is invariant during scanning exposure. It is, however, known that the average of pulse energy of pulsed light output from the pulsed light source varies in the middle or long term, for example during a process of transfer from a certain shot area to a next shot area or during a process of transfer from a certain wafer to a next wafer, though the pulsed light source is normally stabilized by a stabilizing method of emission energy by voltage control of high-voltage power supply.
Accordingly, when the control of exposure dose by the pulse count method was carried out in the projection exposure apparatus of the scanning exposure type using the pulsed light source as an exposure light source, there was a problem that a difference could be made in the average of exposure doses at respective points in a shot area between shot areas or between wafers, thus leaving a possibility that some shot areas were exposed by an exposure dose over a permissible dose.
The above description concerned the cases using the pulsed light source. Similarly, even in the cases of performing exposure by the scanning exposure method using continuous light such as bright lines of the mercury lamp (the i-line etc.), the conventional control method had a possibility that a certain shot area on the wafer was exposed by an exposure dose exceeding the permissible dose because of a change in light amount of the continuous light.